The use of rotating polygon scanning mirrors in laser printers to provide a beam sweep or scan of the image of a modulated light source across a photoresisted medium, such as a rotating drum, is well known. More recently, there have been efforts to use a much less expensive flat member with a single reflective surface, such as a resonant oscillating mirror to provide the scanning beam. Further, resonant oscillating members other than mirrors may also be useful. These resonant scanning devices provide excellent performance at a very advantageous cost. However, because a permanent magnet (drive or sensing) is typically mounted on the back side of the resonant member, the center of mass of the magnet and other rotating elements have very close and critical tolerances.
In addition, the critical mass of the device further complicates the task of maintaining the resonant frequency within acceptable tolerances. According to prior art magnetic drive mechanisms for these oscillating devices, a permanent magnet is mounted to the back side of the resonating device, such as the back side of a mirror surface. This permanent magnet interacts with a drive coil located very close to the device. The critical mass balance of the device requires that the permanent magnet be designed with a size, thickness, and mass having very close tolerances. Other resonant torsional hinged device arrangements may use the permanent magnet as a sensing magnet and use an inertia or piezoelectric drive mechanism to maintain the device or mirror oscillating at its resonant frequency.
However, regardless of whether the magnet is used as a sensing magnet or a drive magnet, it causes problems in maintaining the flatness of the device and significantly increases the oscillating mass. Since a primary use of torsional hinged devices is the scanning mirror in laser printers, flatness and a stable resonant frequency is important. One solution to the problems discussed above is to move the permanent magnet from the back of the oscillating device or mirror onto the torsional hinges. Unfortunately, although this solution solves many of the problems discussed above, it also adds length to the device, which of course makes the device larger and more costly.
Therefore, it would be advantageous to provide an inexpensive and easily manufactured mirror structure that has the advantages of a system wherein the magnets are mounted on the torsional hinges but does not require the extra length of such a structure.